Summary: Evaluation of protective immunity for pneumococcal vaccines involves using reliable serological surrogates of vaccine efficacy. In addition to the quantitative antibody response, immunologic properties including avidity and opsonophagocytic activity may serve as important surrogates. The avidity of polysaccharide (PS) specific antibodies has been found to affect functional activity against many pathogens. Studies of the human antibody response to Haemophilus b PS have implicated avidity as a determinant of antibody bactericidal and protective immunity. Antibodies of high avidity were superior to antibodies of low avidity for their bactericidal activity and protective efficacy towards experimental Hib infection. Since it is known that protective immunity against pneumococcal infection is directly associated with opsonic antibody levels, it is important to determine the quality as well as the quantity of these antibodies. Antibody avidity, which is an expression of the functional antibody affinity, may affect the protective efficacy of antibodies. The objective of the present study was therefore to evaluate in mice the avidity, antibody response and opsonophagocytic activity of serum antibodies to the type 9V PS conjugated to inactivated pneumolysin (Ply) or autolysin (Aly) and to 9V PS in the 7-valent PS-CRM197 conjugate vaccine. The effect of priming mice with the conjugate vaccine followed by a booster dose of the PS vaccine was also investigated. The results of present study demonstrate that the relative avidities of type 9V IgG and IgM antibodies were higher in mice immunized with monovalent 9V PS-protein or 7-valent conjugate vaccine compared to the group that was immunized with 9V PS alone. The serum IgG and IgM antibody responses to 9V PS IgG and IgM antibodies were also higher in mice immunized with the monovalent or 7-valent conjugate vaccine. High IgG and IgM antibodies were maintained for over 12 weeks in mice immunized with conjugates and for over 8 weeks after given a booster dose of 23-valent PS vaccine. Furthermore, the opsonophagocytic activity of antisera from mice immunized with the monovalent or 7-valent conjugate vaccine was higher compared to the PS. These results indicate that monovalent and polyvalent conjugate vaccines can provide effective functional activity that correlates with protective immunity against pneumococcal infection. Higher-avidity antibodies to type 9V PS were more effective than lower-avidity antibodies in mediating opsonophagocytosis. These findings suggests that protective immunity against invasive pneumococcal disease depends on both antibody concentration and functional activity. Thus, evaluation of pneumococcal PS and conjugate vaccines should involve both measures of quantity of serum antibody and its functional activity.